Table/Cell styles can be a little mystifying; here's one of the more straightforward tutorials I've found for those new to working with them: https://www.bookdesignmadesimple.com/making-tables-look-good-in-indesign/ Re: why tables need to be in a text box, InDesign thinks of all content in terms of frames: content is either contained by a frame, or it is a frame. While I don't know the exact technical reason for this, my guess is that this frame-based logic exists to allow you to apply Object Styles to the frames in the question. So, for example, if you always want your tables to be free-floating with text wrapping around them instead of being part of the text flow, you could create an object style that would always apply a text wrap to your table frame, versus you having to constantly duplicate the frame or manually enter text wrap settings to achieve the same affect. This saves time in both creation and updating: if you later decide you want your table frame to always have padding, a stroke, to have its own baseline grid, to have a fill color, whatever-- you can set it in the object style and the change(s) will be instantly reflected across the entire document, versus you having to update the frames one. by. one. (Also, George's answer is correct: select the entire table and set the stroke color to "none." If this is a style you're going to be using frequently, be sure to save it as a table ​and ​cell style as well.)
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